\documentclass{article}
\usepackage{epsfig}
\pagestyle{empty}
\begin{document}
\[ f_o = f_d + \frac{x(f_w - f_d)}{255} \]
\pagebreak

\[ x = 255 \frac{(f_o - f_d)}{(f_w - f_d)} \]
\pagebreak

\end{document}
